Progressive development of the Paleocene to early Miocene Shimanto accretionary complex in eastern Shikoku, Southwest Japan: response to magmatic arc evolution along the East Asian continental margin

Abstract Evolution of the Shimanto accretionary complex suggests implications for the geological history of the East Asian continental margin. To better understand the tectonic evolution of the complex during Paleocene to early Miocene, we collected new petrological and geochemical data, along with U–Pb ages, from sandstones and felsic tuffs in the Murotohanto and Nabae groups in eastern Shikoku. These data clarify the depositional ages and stratigraphic architecture of the two groups, providing insight into the late Paleocene to early Miocene sequence of accretionary processes. Sandstone petrography and geochemistry reveal that the two groups have distinct provenances and demonstrate temporal variations related to the regional tectonic evolution of the continental margin. Sandstones in the Murotohanto Group contain abundant felsic igneous fragments with two detrital zircon age clusters. Cretaceous detrital zircons were supplied by erosion of the Cretaceous magmatic arc during subduction of the Izanagi–Pacific ridge (c. 60–45 Ma). Zircons with variable Permian to Cretaceous ages originated from pre-Cretaceous basement rocks and the Cretaceous arc during subduction of the relatively young Pacific Plate (c. 45–28 Ma). Sandstones from the Nabae Group exhibit petrological and geochemical characteristics indicating the influence of more mafic to intermediate igneous rocks in the source region. Detrital zircons in the Nabae Group are mainly Cretaceous and Paleogene in age, with a small number of early Miocene zircons. These zircons were supplied during erosion of the Cretaceous and Paleogene arcs, which was associated with rifting of the continental margin before opening of the Japan Sea and subduction of the Pacific Plate (c. 28–18 Ma). Transition in provenance occurred in the middle Eocene (c. 45 Ma) and the mid-Oligocene (c. 28 Ma), implying that crustal erosion extended from forearc to back-arc regions. The middle Eocene event was triggered by magmatic arc development after the Izanagi–Pacific ridge subduction, whereas the mid-Oligocene event was related to the timing of crustal uplift with regional unconformity and rifting-related magmatism in Japanese arc.